This has to do with how light-information traces are calculated (bounces and intersections). Simply put, reflection and different types of material will have more light bounces and that causes noise as I understand (I only briefly did search on this some time ago). You can either de-noise it all with Blender’s relatively-new denoiser, or you can attempt to change the materials and location of the lights. The last-mentioned option is the worst of them all. The negative side to de-noising is that it is process-intensive. Another thing is that light information is incomplete. If this is the case, upping the number of render iterations can make it better as the light-path traces are better calculated, but… the number of times that this has to happen can be so large (not always) that you will wait forever.
